A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                              C          ⁢                                          ⁢          D                =                              k            1                    *                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include an excitation beam such as a laser (for instance and infra-red laser) for exciting a fuel to provide the plasma, and a radiation source for containing the plasma. The plasma may be created, for example, by directing a laser beam (i.e., initiating radiation) at a fuel, such as particles (usually droplets) of a suitable fuel material (e.g., tin), or a stream of a suitable gas or vapour, such as Xe gas or Li vapour. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector (sometimes referred to as a near normal incidence radiation collector), which receives the radiation and focuses the radiation into a beam. The radiation collector may have any other suitable form, such as a grazing incidence collector. The radiation source may include an enclosing structure or chamber arranged to provide a vacuum or low pressure environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source. In another system, which may also employ the use of a laser as an excitation beam, radiation may be generated by a plasma formed by the use of an electrical discharge—a discharge produced plasma (DPP) source. Discharge Produced Plasma (DPP) radiation sources generate radiation, such as extreme ultraviolet radiation (EUV) from a plasma formed by means of a discharge, and in particular may involve high temperature vaporisation of a metal fuel for the generation of radiation by directing an excitation beam such as a laser beam towards the metal fuel. Metal, typically in molten form, may be supplied to discharge surfaces of plasma-excitation electrodes and vaporized by means of irradiation with an excitation beam such as a laser beam whereby a high temperature plasma may be subsequently excited from the vaporized metal fuel by means of a high voltage discharge across the electrodes.
The DPP radiation source apparatus may include an enclosing structure or chamber arranged to provide a vacuum or low pressure environment to support the plasma. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, such as a mirrored normal incidence radiation collector, which may form part of the radiation source apparatus. In such a case, the radiation source apparatus may be referred to as a source collector apparatus.
As used herein, the term vaporization is considered to also include gasification, and the fuel after vaporization may be in the form of a gas (for instance as individual atoms) and/or a vapour (comprising small droplets). The term “particles” is used herein includes both solid and liquid (i.e., droplet) particles.
Generation of plasma may result in contamination of the radiation source caused by particulate debris from the fuel. For example, where liquid tin is used as a fuel source, some of the liquid tin will be converted into a plasma, but particles of liquid tin may be emitted at high speeds from the plasma formation location. Such fuel particles are referred to herein as primary debris particles. The liquid fuel particles may solidify on other components within the radiation source, affecting the ability of the radiation source to generate a radiation producing plasma or to provide a beam of radiation from the plasma. In order to reduce or hinder contamination of optically-active surfaces within the radiation source, by the primary debris, debris-receiving surfaces may be positioned within the radiation source to deflect or capture such primary debris particles. In this specification, the term “optically-active” is merely used to denote surfaces which have an optical role to play, such as mirrors, lenses, viewing ports, sensors and the like, and is not meant to imply any optical activity in terms of the modification of the optical axis of polarised radiation (which is understood to be an alternative meaning of the term “optical activity” in the art).